In vehicles which run on a diesel engine, it is well-known that particulate matter contained in the exhaust gas of the engine is trapped by a diesel particulate filer (DPF) provided in the exhaust pipe to prevent the particulate matter from being discharged into the atmosphere. The particulate matter trapped by the DPF is burned by means of an operation to raise the temperature of the DPF, and thus removed from the DPF. This operation is known as DPF regeneration.
JP2002-168112A, published by the Japan Patent Office in 2002, proposes that the oxygen concentration of the exhaust gas be increased during DPF regeneration to promote combustion of the particulate matter. More specifically, the oxygen concentration of the exhaust gas is regulated in accordance with an exhaust gas pressure differential upstream and downstream of the DPF, and the elapsed time from the start of DPF regeneration processing.